But since Keeler's discovery there has been a decided turning away of
speculation another way. The form of the spiral nebul? seems to be
entirely inconsistent with the theory of an originally globular or
disk-shaped nebula condensing around a sun and throwing or leaving off
rings, to be subsequently shaped into planets. Some astronomers,
indeed, now reject Laplace's hypothesis in toto, preferring to think
that even our solar system originated from a spiral nebula. Since the
spiral type prevails among the existing nebul?, we must make any
mechanical theory of the development of stars and planetary systems
from them accord with the requirements which that form imposes. A
glance at the extraordinary variations upon the spiral which Professor
Keeler's photographs reveal is sufficient to convince one of the
difficulty of the task of basing a general theory upon them. In truth,
it is much easier to criticize Laplace's hypothesis than to invent a
satisfactory substitute for it. If the spiral nebul? seem to oppose it
there are other nebul? which appear to support it, and it may be that
no one fixed theory can account for all the forms of stellar evolution
in the universe. Our particular planetary system may have originated
very much as the great French mathematician supposed, while others
have undergone, or are now undergoing, a different process of
development.
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